Continuing pressure on the limited radio spectrum available for radio communication systems is focusing attention on the development of spectrally efficient linear modulation schemes. Recent studies of digital mobile radio systems propose the use of linear modulation methods such as QPSK and .pi./4 QPSK which are considered to improve spectrum efficiency. The schemes, however, require a highly linear power amplifier or distortion will lead to intermodulation products, i.e. adjacent channel interference or "splatter". In the field of private mobile radio, stringent restrictions are placed on out-of-band emissions.
There is a demand for a strict linearity performance in power amplifiers.